Two earthquakes shook Mona Epstein awake in the middle of the night, long before she crawled out of bed on the morning of Dec. 8.
But it was the rocking from the magnitude 3.6 quake just after 9 a.m. that caused the San Ramon resident to scream.
“There was a loud rumble,” said Epstein, who lives about a mile from where the quake hit. “The cupboard doors opened, my armoire door popped open and things popped out of the closet.”
Epstein said she hesitated to even shower afterward for fear of another earthquake. “If it was the big one,” then she didn’t want to “be naked and have to run out.”
Nearly 1,600 people as far away as San José reported they felt the quake, which occurred along the Calaveras Fault, the United States Geological Survey reported.
Over the last month, more than 150 earthquakes jolted the San Ramon area — including 10 earthquakes at or above a magnitude 2.5 on Dec. 8, according to Amy Williamson, a research seismologist at the UC Berkeley Seismology Lab.
This part of Contra Costa County is prone to earthquake swarms and has experienced these events a handful of times since the 1970s, due to a complex system of faults.
“They’ve had these sorts of swarms for decades now,” Williamson said. “For San Ramon to the Danville area, it’s really common.”
Epstein experienced an earlier swarm back in 2018 while living in San Ramon, a city with a population of nearly 80,000.
While these earthquake clusters aren’t out of the ordinary, they can still come as a surprise, especially if you live right above the jolt, like Rachael Heys, whose street in San Ramon is located right over the epicenter of last week’s swarm.
She credits her cat, Marshall — named after rapper Eminem — for warning her that an earthquake was about to hit in the middle of the night. He made “weird little noises” and hid under a table.
“Within seconds of him yelling this big loud meow, there was a big earthquake,” Heys said. “It really shook me. It sounded like a dresser hit the wall. It was like this one big bang.”
Heys and her boyfriend felt at least two other quakes that morning.
“We had all of these mini ones mostly in that one day,” Heys said. “This is insane.”
Earthquake swarms differ from a typical earthquake sequence, where there’s one main shock and then a series of small aftershocks, Williamson said. Swarms don’t usually have a dominant earthquake. Instead, a cluster of tiny or minor earthquakes takes place over a more extended period of time, and then the fault quiets down.
“The fact that there’s a swarm here doesn’t make it any more or less likely for the big one that people are always talking about in the Bay Area,” Williamson said.
Earthquake swarms typically occur due to changes in the liquid around a fault. They’re common in places near volcanoes and geothermal areas. But Williamson said what makes the San Ramon area “a little bit unique” is that the area isn’t volcanic or hydrothermal.
San Ramon sits over a complex geologic environment, which is one theory, Williamson said, for why the area gets swarms every few years. The Calaveras Fault runs underneath the city in a transition zone.
It is part of the San Andreas Fault system and is capable of a magnitude 6.7 earthquake. To the east, the Mount Diablo Thrust Fault begins.
“All these earthquakes happen in that transition area,” Williamson said. “Any small changes kind of cause that area to preferentially get more earthquakes.”
The last big quake the Calaveras Fault produced was in 1984 in Morgan Hill with a magnitude of 6.2. But if the Hayward Fault and the Calaveras Fault, which UC Berkeley scientists found are connected, rock simultaneously, that could result in a magnitude 7.3 earthquake.
Faults contain liquid that lubricates the earth, making it easier to move and causing earthquakes. Similar to a person wetting their hands and sliding them together, the fluid reduces the friction, and the rock “can slide more freely,” Williamson said.
Williamson said the liquid is usually a mix of water and minerals, and that there isn’t a “great model” to say exactly how it moves through cracks within the fault system. That movement can cause the quakes.
Williamson said the “constant rattle” should serve as a reminder that Bay Area residents live in a “really seismically active area.” She recommended preparing a go bag with clothes, food and water.
Epstein, who is retired, said she’s started to prepare for a larger earthquake, collecting masks, gloves, water, tools, and canned food — although she needs to double-check the expiration dates to make sure the food is still good to eat.
“I’m not to the point where I’m gonna sleep with my shoes on or anything,” Epstein said. “I just hope if the worst happens that I can get to the bag in the closet.”
Heys, who works as a server in Danville, on the other hand, is very prepared. She has a supply of perishable food, flashlights, portable chargers that also act as flashlights, mini candles, gallons of water and a case of water in her car.
She’s talked with her friends and neighbors about the quakes and how they’ve prepared, but they don’t seem as concerned.
“Nobody really has any major concerns as of now,” Heys said. “I think that’s because none of us has experienced a huge one yet.”
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